Degenerative amplifier circuit



Nov. 12, 1940. w 51x 2,221,116

DEGENERATIVE AMPLIFIER CIRCUIT Filed April 19, 1952 INV EN TOR.

/ EM SIX ATTORNEY.

Patented Nov. 12, 1940 1. :;1-:-:...

UNITED V DE ENERATIVEAMPUHER cmo'urr f i nwillem Six,"Eindhoven Netherlands, assignor, by i 23:; ';-mesne assignments; to Radio Corporation 10f; 1;

America, Ncw York, N. Y., a corporation of Del-;

' Application Aprillll,1938,'SerialNol 202;825 11 I T jIn G ermany May 24,1937 r:

rena e. (01. 179 1 This invention relates 'to amplifiers provided with degenerative back-coupling in orde r to de crease the non-linear deformation occurring-in the amplifier. a

As is well-known in such amplifyingarrangements a voltage is taken off from the'anode circuit of an amplifying tube, which voltage is supplied back through a back-coupling'circuit to the grid circuit of the same tube, o'r'of' a' preceding amplifying tube. In the optimum 'case the phase difierence between this voltage and the Under certain conditions, however; positive back-"- coupling instead of degenerative back-coupling may occur, by which positive back-coupling the amplifier may be caused to oscillate, which will be explained-with reference to; Figs. l gand 2 of the accompanying drawing.

In the drawing:

Fig. 1 shows schematically afnegative feedbackcircuit for the purpose of analysis,

Fig. 2 is a graphic analysis of the action for the feedback circuit of Fig. 1, 4 I j Fig. 3 is a graphic analysis of the problem sought to be solved by this invention, Fig. 4 illustrates a circuit embodying' theinjvention. J i Fig. 1 represents an amplifier V. comprising input terminals I and 2, to Which'iSl supplied the Voltage e1 to be amplified, andoutput terminals 3 and 4 between which occurs the amplified voltage 62. By means of a potentiometer consisting;

for instance, of two impedancesjZr and Z2 a voltage ek=l362 is taken off from theoutputcir-Z cuit of the amplifier and supplied back through the connecting lines 5 and 6 to the input circuit of the amplifier. When the amplification of theamplifier without back-coupling is p, the amplification ,u with back-coupling is given by the relation:

Generally ,a' and p and consequently also the product 5 are complex values which are dependent on the frequency and may be represented by vectors. The locus of the end point of the vector 3 for all frequencies between 0 and 00 will then be given, for instance, by the curve A shown in Fig. 2, which is so constructed that the horizontal coordinate measured on the X-axis and the perpendicular coordinate measured on the Y-axis of any point of the curve respectively correspond to the real and imaginary part of the complex value s respectively. -It fu 'rther follows "that the complex value-'l-l-jlp' for any frequency is given b'y' the vectors drawn "from' the point on the horizontal axis with the coordinate 1 -to the periphery "ofthelc'urve .A. '1 .1

-Nyquist (see Bell System Technical Journal, Jan. 1932, pages 126147) has shown that the amplifier will not "oscillate 'at any frequency if the curveAdoes'not include the point 1 onthe horizontal axis. In: other words the point --1 should not lie inside the plane enclosed by the curveA. i Nowthis condition is not. satisfied when in theiamplifier two transformers are connected between theipoints from which isderived the voltage back-coupled in ardegenerative manner and. the points-to which this voltage is supplied back-1T0 explain this 'more. clearly-the ratio between the. secondary voltage and the primary voltage'of-a transformer for allhfrequencies" be-; tween0 and v(this. ratio may be represented. by a complex value v) is represented :by the curve B of Fig-.3v which similarly to the curve A shown in Fig-231s constructed. so that the coordinates of-anypoint of the curve measured on the X and the Y-axisindicate the real and imaginary part respectively of the ratio 1;. jv '1.

Transformers used in amplifiers are proportionedsolthat the absolute value of the voltage ratio .12 in the range of frequencies to be uni formly amplified'is about constant. In Fig. 3 the lowest frequency'cf this range is represented by If and th'ehighest frequency coinciding with theleakage resonance frequency of the transformer byxfz. 1' Consequently the length of the vector v is substantially "constant between these limits,. whereas for the "frequencies higher than fzthelength of '12 rapidly decreases. Furthermore, it appears from Fig. 3 that the phase displacement between the primary and the secondary voltage amounts to 90 at the leakage resonance frequency.

In view of this behavior of a transformer it will be appreciated that self-oscillation of the amplifier with sufficient degenerative back-couthe plane enclosed by the curve A in that the amplifier will oscillate.

In Fig. 4 the amplifier B1 has the input voltage e1 applied between input terminals I and 2. The transformer T1 couples the plate circuit of B1 to the input electrodes of the following amplifier B2. Transformer T2 is schematically represented, but it is to be understood as comprising a split primary winding. One section P1 of the primary has its upper end connected to the plate of amplifier B2, while its lower end is connected to the positive terminal of the direct current source (not shown). The second primary section P2 Fig. 2 so has one end connected to the grounded end of the current source, while the opposite end thereof is connected to the cathode leads of tubes B1 and B2. Resistor R connects the cathode of amplifier B1 to ground. It will be noted that'the secondary winding S feeds the output voltage e2 to terminals 3 and 4. The direct current source, it will be seen, is in series relation between the primary sections P1 and P2. The alternating voltage across winding P2 is impressed between the input electrodes of B1. The problem is to accomplish the degenerative feedback without oscillation production.

This difiiculty is obviated when, according to the invention, the leakage resonance frequency of one of the two transformers T1 and T2 eX- ceeds the leakage resonance frequency of the other transformer and the range of frequencies to be amplified is limited by the lowest leakage resonance frequency. In this case the total phase displacement caused by the two transformers in the range of frequencies to be amplified is less then 180 andsince the voltage transmission of the transformer with a low leakage resonance frequency rapidly decreases for frequencies exceeding the leakage resonance, the absolute value of l+,u,8 will also rapidly decrease for these frequencies. In this manner it is possible by a suitable choice of the leakage resonance frequencies to give the curve A in Fig. 2 traced by the vector 1|-,u,8'Sl1ch a shape that the point -1 is not enclosed by the curve, solthat the amplifier cannot oscillate.

What is claimed is:

1. In an amplifier for uniformly amplifying alternating voltage of a range of frequencies, voltage input terminals and output terminals, said amplifier comprising a pair of cascaded tubes, the first ofthe tubes having its input electrodes connected to said input terminals, a transformer coupling the output electrodes of the first tube to the input electrodes of the second tube, a second transformer coupling the second tube output electrodes to said output terminals, said second transformer including a section thereof coupled to said first tube input electrodes to feed back voltage in degenerative phase, said transformers having leakage resonance frequencies which are sufiiciently different to prevent said feed back causing oscillation.

2. In an amplifier for uniformly amplifying alternating voltage of a range of frequencies, voltage input terminals and output terminals, said amplifier comprising a pair of cascaded tubes, the firstof the tubes having its input electrodes connected to said input terminals, a transformer coupling the output electrodes of the first tube to the input electrodes of the second tube, a second transformer coupling the second tube output electrodes to said output terminals, said second transformer including a section thereof coupled to said first tube input electrodes to feed back voltage in degenerative phase, said transformers having leakage resonance frequencies which are sufilciently different to prevent said fee d back causing oscillation, one of said transformers having its said leakage resonance frequency located substantially beyond the highest frequency of said frequency range.

3. In an amplifier for uniformly amplifying alternating voltage of a range of frequencies, voltage input terminals and output terminals, said amplifier comprising a pair of cascaded tubes, the first of the tubes having its input electrodes connected tosaid input terminals, a transformer coupling the output electrodes of the first tube to the input electrodes of the second tube, a second transformer coupling the second tube output electrodes to said output terminals, said second transformer including a section thereof coupled to said first tube input electrodes to feed back voltage in degenerativephase, said transformers having leakage resonance frequencies which are sufficiently different to prevent said feedback causing .oscillation both said leakage resonance frequencies being located at the upper end of said frequency range, and the lower of the leakage frequencies limiting said. range.

4. In an alternating voltage amplifier network of the type including at least two electron discharge tubes coupled in cascade by a transformer, an output transformer coupled to the second tube output electrodes, and a degenerative voltage feedback connection between said output transformer and the input electrodes of the first tube; the improvement which comprises said coupling and output transformers having sufficiently different leakage resonance frequencies to prevent the production of oscillations in said amplifier.

WILLEM SIX. 

